Transmission of data at high speeds, for instance in Asymmetric Digital Subscriber Line (ADSL) or Very high-speed Digital Subscriber Line (VDSL) networks, commonly referred to as xDSL networks is subject to errors injected by external sources. One particular problem is the influence of crosstalk between various subscriber lines within close vicinity. Crosstalk is the effect whereby a first subscriber line induces signals on a second, different subscriber line which act as noise on that second subscriber line. Such effects are especially noticed when multiple subscriber lines form part of the same cable or cable binder or when they are terminated close to each other, for instance in a Digital Subscriber Line Access Multiplexer (DSLAM) where multiple subscriber lines are terminated on the same board.
Noise effects can be dealt with in various ways, for instance the use of error correction codes can reduce the effects of noise. Such error correction codes, for instance Reed-Solomon, consist of additional bits which are added to the transmitted user data and which can be used by a receiver to detect and eventually correct corrupted data. Generally error codes can only recover a limited number of errors in data but are able to detect more errors. In such case, the receiver may rely on other mechanisms to obtain corrected data, for instance retransmission. Retransmission can be implemented explicitly, whereby a receiver asks the transmitter to resend a piece of data, or implicitly whereby the transmitter waits for a confirmation of the receipt of the correct data and if no confirmation is received, the transmitter will automatically retransmit the data. However error corrections codes and retransmission occupy additional bandwidth, hence reducing the bandwidth available for user data transmission. As such, these error codes reduce the effective speed of high bandwidth connections such as xDSL links. This could be acceptable to cope with impulse noise errors but may become a problem for long term noise effects such as upcoming subscriber lines which may take some time to initialize and establish a connection, and which may permanently induce crosstalk once they are active.
To improve this situation and thus reduce the corrections that are needed, solutions which reduce the effects of noise and more in particular crosstalk are used. Seamless Rate Adaptation (SRA) is a solution which aims at reducing the influence of crosstalk on active lines when a subscriber line is activated. SRA is described in chapter 10.2 of the ITU-T Standard Specification G.992.3 entitled “Asymmetric Digital Subscriber Line transceivers 2 (ADSL2)”. The SRA solution reduces the upstream and downstream bit rate on active subscriber lines in order to make them less sensitive to crosstalk. However, SRA requires communication between the endpoints of the subscriber line to set the low bit rates in upstream and downstream direction. Such communication runs over the same subscriber line as the user data and is thus subject to the same noise effects and error injection. In addition, SRA might reduce the bit rates on active subscriber lines. This means that SRA unavoidably might affect the service of active subscriber lines when a subscriber line is activated.
Similar to SRA is Dynamic Rate Adaptation (DRA) which is described in detail in Appendix II to the ITU-T Standard Specification G.992.1 entitled “Asymmetric Digital Subscriber Line (ADSL) transceivers”. Therein it is mentioned that DRA is designed to cancel impulse noise effects that could cause a short term service interruption of a few tens of milliseconds. Thus, DRA is not designed to deal with long term noise effects which may for instance arise during the initialization of a subscriber line.
Another solution to overcome crosstalk effects during the initialization of a subscriber line is to apply artificial noise, for instance as described in European Patent Application EP 1 641 173 titled “Multi-Carrier modern transmitter with controlled transmit signal quality degradation for improving stability of operation.” or virtual noise on active subscriber lines. This solution consists of injecting noise on active lines representative for the noise of external sources onto the subscriber line. The amount of noise that is injected is an estimation of the noise effects which will be induced on the active subscriber line. However, such estimations are difficult to make. For instance, impulse noise may temporarily induce more noise on the active subscriber line. In addition, artificial or virtual noise reduces the amount of available bandwidth on the subscriber line during times of low noise and may be too drastic to be a viable or acceptable reduction for long term connections.
An example of existing solutions is disclosed in Cioffi et al. US20060274893 titled “DSL system training”. It discloses a system to train new subscriber lines such that the training is non-disruptive to other subscriber lines and which is based on reducing power and measuring crosstalk levels when a new line is inserted. In this particular example the joining subscriber line is already transmitting data (i.e. active) which means that some pre-compensation is required to avoid disturbing other subscriber lines. This pre-compensation requires configuration of the other subscriber lines which may delay the initialization of the joining subscriber line and which requires an a priori knowledge or estimation of crosstalk effects. The transmit power will be gradually increased in a process of successive retrains. Therefore, several service interruptions can occur and the desired data rate may only be achieved after a lengthy procedure.
It is an object of the present invention to provide a device and method for enabling the a priori estimation and mitigation of the future effects of crosstalk induced by an inactive subscriber line which overcomes the shortcomings of the above described prior art solutions. It is another object of the present invention to enable crosstalk compensation mechanisms to operate in scenario's where the activation of an inactive subscriber line is involved, i.e. enabling efficient long-term crosstalk compensation mechanisms with minimum impact on the effective bandwidth and speeds on already active subscriber lines. It is another object of the present invention to enable full speed activation of an upcoming inactive subscriber line.